The role of the Spitsbergen shear zone in determining morphology, segmentation and evolution of the Knipovich Ridge

In 1989-1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km(2) of the seafloor in the Norwegian-Greenland S ea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from i ts oblique (115 degrees ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determin e whether or not the ancient Spitsbergen Shear Zone continued to play any i nvolvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Rid ge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V -shaped-oriented flank faults which have their apex at the present day Moll oy Transform, and rift related faults that extend north of the present Moll oy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial h ighs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi statio nary, focused magma centers creating (axial highs) located between long obl ique rift basins. Present day segment discontinuities on the Knipovich Ridg e are aligned along highly oblique, probably strike-slip faults, which coul d have been created in response to rotating shear couples within zones of t ranstension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formatio n of en-echelon pull apart basins. The curved stress trajectories create ar cuate faults and subsiding elongate basins while focusing most of the volca nism through the boundary faults. As a result, the Knipovich Ridge is chara cterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple she ar detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbe rgen Shear Zone on the evolution of the Knipovich Ridge is the primary reas on that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along th e Knipovich Ridge and Mohns Ridge (280 degrees vs. 330 degrees, respectivel y) cause the crust on the western side of the intersection of these two rid ges to buckle and uplift via compression as is evidenced by the uplifted we stern wall province and the large 60 mGal free air gravity anomalies in thi s area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading r egime has not evolved along this ridge.